1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device and a method of fabricating the same.
2. Background of the Related Art
Generally, a liquid crystal display device includes two glass plates, i.e., a bottom plate and a top plate, in which a liquid crystal is injected therebetween. A thin film transistor and a pixel electrode are arranged on the bottom plate. The top plate has a color filter layer for displaying a color, a common electrode, and a black matrix layer. A conventional liquid crystal display device will be described below with reference to the accompanying drawings.
FIG. 1 is a circuit diagram of a conventional liquid crystal display device. Referring to FIG. 1, a plurality of gate lines are disposed in one direction at a predetermined interval and a plurality of data lines are disposed perpendicular to the gate lines at a predetermined interval.
A plurality of thin film transistors are formed between the respective gate line and the respective data line in a matrix arrangement so as to apply signals of the data lines as well as the gate lines to the pixel electrode. A stacked storage capacitor and a liquid crystal capacitor having a liquid crystal layer as a dielectric are formed in the pixel electrode.
FIG. 2 is a layout illustrating a bottom plate of a conventional liquid crystal display device. FIGS. 3A-3H are sectional views illustrating fabricating process steps of a liquid crystal display device taken along I--I line of FIG. 2.
As illustrated in FIG. 3A, polysilicon is formed on a transparent insulating substrate 1 of glass or quartz and patterned to form an island shaped active layer 2. The active layer 2 serves as an active region of the thin film transistor and an electrode of the storage capacitor.
As illustrated in FIG. 3B, a photoresist 3 is coated on an entire surface of the substrate 1 and patterned to expose the active layer 2 which will be a lower electrode of the storage capacitor. Impurity ions such as phosphorus P or boron B are implanted into the exposed active layer 2 using the photoresist 3 as a mask.
Subsequently, as illustrated in FIG. 3C, the photoresist 3 is removed and a gate insulating film 4 is formed on the entire surface of the substrate 1 including the active layer 2. Polysilicon containing impurity ions is deposited on the entire surface of the substrate 1 including the gate insulating film 4. Silicide material such as WSix or MoSix is then deposited thereon and patterned to define gate electrodes 5 and 5' and upper electrodes 5a and 5a' of the storage capacitor. Here, the gate electrodes 5 and 5' are used as the gate line and the upper electrodes 5a and 5a' are used as the common electrode.
The impurity ions such as phosphorus or boron are implanted into the active layer 2 using the gate electrodes 5 and 5' as masks and the impurity ions are activated by heat treatment to define source and drain regions of the thin film transistor.
As illustrated in FIG. 3D, a first interlayer insulating film 6 is deposited on the entire surface of the substrate 1 including the gate electrodes 5 and 5'. A first contact hole 7 is formed by selectively removing the gate insulating film 4 and the first interlayer insulating film 6 to expose the source region of the active layer 2.
As illustrated in FIG. 3E, a metal is deposited on the entire surface of the substrate 1 including the first interlayer insulating film 6 and patterned so that a data line 8 is formed to be connected with the active layer 2 through the first contact hole 7.
As illustrated in FIG. 3F, a second interlayer insulating film 9 is deposited on the entire surface of the substrate 1 including the data line 8. A second contact hole 10 is formed by selectively removing the gate insulating film 4 and the first and second interlayer insulating films 6 and 9 to expose the drain region of the active layer 2.
As illustrated in FIG. 3G, a transparent conductive material such as Indium Tin Oxide (ITO) is deposited on the second interlayer insulating film 9 and patterned. A pixel electrode 11 is then formed to be connected with the active layer 2 through the second contact hole 10. The active layer 2 is hydrogenated to remove its defects.
As illustrated in FIG. 3H, a protective film 12 is formed on the entire surface of the substrate 1 including the pixel electrode 11 and a pad is open. As a result, the lower plate of the liquid crystal display device is completed.
As aforementioned, the conventional liquid crystal display device has several problems.
First, since the storage capacitor has an opaque stacked structure of the active layer, the gate insulating film, and the gate electrode, aperture ratio is reduced as much as an area 15-25% of the pixel region due to the storage capacitor.
Second, since the suicide material such as WSix or MoSix is deposited on the polysilicon containing the impurity ions to be used as the gate electrode material, there are problems in that the step of forming the suicide material is complicated and resistance becomes larger than that of the metal material.
Third, in the conventional liquid crystal display device, the photoresist is patterned on the active layer and the impurity ions are implanted into the active layer to define the lower electrode region of the storage capacitor. In this case, there are problems because the step of removing the photoresist is difficult and the surface of the active layer is damaged during that step of removing the photoresist, thereby deteriorating characteristics of the device.
Finally, for the case in which the silicide material such as WSix or MoSix is used as the gate electrode material, hydrogen ions are laterally-diffused through the gate insulating film instead of being diffused into the channel region through the gate electrode during the hydrogenation process. There results in problems that it takes much time during hydrogenation process and performance of the device is reduced.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.